shell: Fix clang warnings about "string plus integer"

shell: Fix clang warnings about "string plus integer"

Building with clang results in some warnings about integer values being
added to strings:

-----------------------------------------------------------------------
eval.c:1138:13: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
                p = " %s" + (1 - sep);
                    ~~~~~~^~~~~~~~~~~
eval.c:1138:13: note: use array indexing to silence this warning
                p = " %s" + (1 - sep);
                          ^
                    &     [          ]
1 warning generated.

...

jobs.c:1424:16: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
                        str = "\"}" + !(quoted & 1);
                              ~~~~~~^~~~~~~~~~~~~~~
jobs.c:1424:16: note: use array indexing to silence this warning
                        str = "\"}" + !(quoted & 1);
                                    ^
                              &     [              ]
1 warning generated.
-----------------------------------------------------------------------

While the code itself is fine and the warnings are indeed harmless,
fixing them also makes the semantic more explicit: what it is actually
being increased is the address which points to the start of the string
in order to skip the initial character when some conditions are met.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>